A. Herr, C. Clark, F. S. Anderson, and D. T. Anderson

1. Wall Conditioning for 1T Operation in HSX A. Herr, C. Clark, F. S. Anderson, and D. T. Anderson HSX Plasma Laboratory, University of Wisconsin – Madison Overview Carbonization Boronization Impurity radiation • 1T operation in HSX shows high impurity influx • High radiated power • O, Fe spectral lines are larger than in 0.5T operation • Density ramps regardless of puff • Attempted solutions • Carbonization • Lower radiated power and line radiation • Controllable density • Repeatable day to day but changes during runday • Probe limiter • Stops ramping density • Not repeatable or controllable enough for day to day use • Boronization • Preliminary results only • Benefits of boronization not yet realized • All data shown is QHS configuration with 44kW ECRH Method: Method: • o-carborane precursor (used on TJ-II and L-2M) • Experiments performed with one injection oven and four ovens • o-carborane heated to 100 °C in closed oven • All ovens opened to vessel during He GDC • ~40% o-carborane glow • Followed with pure He GDC to desorb H • No vessel baking • 1 hour carbonization before each runday • 2mTorr CH4 admixed with 3mTorr He glow • Followed by 3 hour He GDC Results: • Steady state density reliably achievable between 1.5x1012 and 5x1012cm-3 • Radiated power increases through day • Less puff needed to maintain density during day – indicative of increased recycling • Short He glow resets nominal recycling rate and reduces radiated power • Boron emission appears after boronization – boron is in the machine • Carbonization reduces oxygen emission more than attempted boronization – boronization has not yet succeeded Summary Results: • Collection probes show very non-uniform deposition • One oven (1g) showed ~150nm on probe C, nothing on the others • Four ovens (2g total) showed ~420nm on one probes A and D and ~50nm elsewhere • No noticeable change in plasma performance After four oven boronization probe C oven 1 • 1T operation in HSX requires wall conditioning beyond He GDC used for 0.5T • Wall conditioning with carbonization has been the most successful at giving reproducible plasmas and control from 1-5x1012cm-3 • Limiting wall interaction using a probe limiter was successful over a smaller range of plasma parameters than carbonization • Boronization of HSX is a work in progress • Deposited boron in the machine • Toroidally non-uniform film thickness observed • Does not reduce oxygen emission • No visible effect on plasma performance probe B Limiter location Original 1T operation oven oven probe D probe A 0.5T 1T oven Probe limiter H glow on B Method: Method: • ¾” graphite rod inserted on low field side near midplane • Grounded to vessel • Most effective when inserted to r/a~0.74 • No wall treatment • Begin day with H glow • Increase H inventory • Strong wall sourcing provides predominantly H plasma rather than impurity plasma 44kW injected Future possiblities Results: Results: • Allowed steady state density above 4x1012 • Radiated power increases while puff decreases through shot – implies impurity fueling • Not reliable enough for everyday use • Difficult to obtain steady state density • Steady state possible above ~4x1012 • Better shot-to-shot repeatability than carbonization • Unpredictable day-to-day • Large recycling • Radiated power shows considerable impurity content • More experiments with o-carborane boronization • Design of safe trimethylboron system • Carbon tiles in high flux regions of vessel • Limiter with larger plasma facing area • Steady state density 0.2-2x1012 • Density ramps to cutoff regardless of fuelling • Radiated power follows density rise • He GDC not good enough • Radiated power <10kW • Only He GDC only wall conditioning needed